A camera at it's most basic level involves 3 things: shutter, aperture, and ISO. In my next few posts, I'm going to try and explain each of them in the easiest way that I can. Today, let's focus on the shutter.
The easiest way to think about photography basics is pretending you have an actual film camera in hand. When film is exposed to light, or a scene, that light is burned into the grain of the film. When this happens, those grains take on the image and when processed, take on the color negative that we get our pictures from.
The shutter is always closed, to prevent the "film" from getting exposed to the light. When we open the shutter, that's when the picture is taken and grains are exposed. If the shutter is open for a shorter amount of time, the less information the grain takes on. If the shutter is open for a longer amount of time, the more information the grain takes on. If you want less motion, for instance a car in a race, you would want the shutter open for a shorter amount of time to catch the exact moment and that's it(lets say 1/8000). If you wanted to catch the car in the race and the trail he leaves, you would leave the shutter open longer to catch more information(lets say 1/8).
The best way to describe this is for you to shut your eyes. Actually, read this first, then do what I say. Shut your eyes. Open and close them as quick as you can. When you're done, what do you remember of the scene? Only that split second right? Now, shut your eyes, and leave them open for longer then close them. Now what do you remember? Maybe a dog walked in front of you or someone sat a cup of coffee down on the table. This mimics a short shutter speed(1/8000) and a long shutter speed(1/8 etc). The longer you leave the shutter open, the more motion/information you'll catch of the scene.
Wonder how people catch light trails on dark nights on the road? Long shutter speed. Ever see a picture of a hummingbird hovering still in a photo? Extremely fast shutter.
Hope this helps. Aperture is up next... and that one will be a little tougher.